Because there are a wide variety of applications for mechanical packing and seals, including packing for pumps, valves, hydraulic and pneumatic equipment, a whole industry has grown up in their design and construction. In the areas with which the present invention is concerned, the packing is generally sold in relatively long coils of braided packing material of a square or rectangular cross section from which many suitable lengths may be cut. Conventionally, several lengths are cut from a coil of material of an appropriate cross sectional dimension for a given installation, each length being formed into a ring about a shaft, with the cut ends abutting each other. Often a half dozen or more rings are disposed about shaft with their sides in abutting relationship.
In a typical situation, a so-called packing or stuffing box surrounds the shaft. The interior of the stuffing box is of a diameter sufficiently greater than the shaft to accomodate the packing rings. An annular gland is fitted about the shaft and bolted to the exterior of the stuffing box in such a fashion that an end of the gland compresses the packing rings in the stuffing box. Tightening the bolts pulls the gland toward this housing and compresses the packing rings within the stuffing box. Under such compression, the packing tends to expand radially to some extent and substantially fill the stuffing box to prevent or minimize the escape of the contents of the equipment at the intersection of the shaft and housing.
Modern fiber manufacturing techniques have increased the number of materials available for the construction of packings. These new fibers have been made into so-called synthetic yarns that are in turn braided into packings which have increased heat tolerance and chemical resistance when compared to the natural fibers such as flax or cotton and metallic packings such as lead or aluminum or which are less hazardous when compared to mineral fibers such as asbestos, that were the norm up to about 1960. These newer synthetic materials include, but are not limited to, acryllic, aramid, carbon, glass, graphite and polytetrafluoroethylene (PTFE). All have been braided both alone and in combination with a variety of liquid and/or solid impregnants to form mechanical compression packings for both general and specialized applications. One of the newest popular yarns combines a PTFE matrix expanded about graphite particles and a so-called inert break-in lubricant, silicone oil, in tape that is folded into a yarn form. This material, manufactured by W. L. Gore & Associates is called GFO fiber.
Despite these advances, several well known problems exist with the basic stuffing box arrangement. One of these problems concerns extrusion of the packing into the small clearance between the bottom, or throat, of the stuffing box and the shaft and also between the gland and the shaft. This occurs if the pressure in the stuffing box is high enough to overcome the shear strength of the materials used in the packing, and is the result of an excessively large clearance at either location, over-tightening the gland bolts, excessively high seal water pressure, or a combination of these factors. When extrusion occurs, the friction of the packing increases, causing unwanted heat, dry running of the packing-shaft interface, and increased wear both of the packing and the shaft surface. There is also an increase in the power required to drive the equipment. Usually extrusion results in failure of the packing which is exhibited by uncontrolled leakage shortly after the extrusion occurs.
Another problem present in certain hydraulic cylinders that are used in presses and that depend upon gravity to lower the cylinder, is friction. Loads on the packing required to restrict leakage may be so high as to prevent or retard the return of the cylinder to its bottom position. To solve this problem, some packing manufacturers have created molded rings having hydraulically augmented sealing loads on the lips of these seals that contact the cylinder and stuffing box wall. This is expensive as individual molds may be required for each size of cylinder and stuffing box cross-section. Further, as many as three different mold shapes may be required to make a single set of these so called V ring packings if they are not installed in a continuous ring.
As a solution to the problem of extrusion and the prevention of so-called rolling in the stuffing box exhibited by some of the modern synthetic fibers when they are loosely braided or packed in the stuffing box, one manufacturer, New England Braiding Company, Inc., has developed and marketed a packing called Superpack 104 Bumblebee Packing. This packing is manufactured in numerous cross-sections on interlocking lattice braiding machines having 3 tracks and 20 carriers for smaller cross-sections and 4 tracks and 36 carriers for larger cross-sections. It combines two different yarn materials. One material, an aramid fiber monofilament containing PTFE and inert oil, such as silicone oil, as impregnants is made by E. I. du Pont de Nemours & Company, Inc. of Wilmington, Del. and is sold under the trade name of Kevlar IT packing yarn. This secondary yarn, appropriately plied to yield a given crosssection is braided from the yarn carriers following both corner or "x" tracks of the interlocking braiding machine upon which it is made. The primary yarn is the aforementioned GFO fiber yarn made by W. L. Gore and Associates, Inc. of Elkton, Md. Again, appropriately plied, this yarn is braided from yarn carriers following the inner or diamond track or tracks. There is one such track on the 3 track machine and two on the 4 track machine. The finished braid has the surface appearance of a series of lopsided Ns for the 3 track machine or "Ws" for the 4 track machine, each having yellow-colored corners and a gray-black color for the inner braid. Thus, all corners are reinforced, which while helpful, presents twice the amount of high strength yarn necessary to the surface of the shaft or rod which greatly increases unwanted frictional resistance and shaft or rod wear. Sometimes use of this yar prevents use in gravity return rams, as will be discussed.
Note center fill material of either or both yarns is used as appropriate to obtain the proper size and, as described in U.S. Pat. No. 4,550,639, included herein by reference, with appropriate warp placements can create a braid which resists "keystoning". This anti-keystoning packing has been found through field testing in the paper industry to resist the type of extrusion found with single-yarn packings.
While the anti-keystoning technique of the above patent impedes unwanted extrusion, the extrusion problem can sometimes exist because of yet another problem. The chemical resistance of the aramid fiber yarn, Kevlar IT is not as broad as that of the PTFE/graphite composite yarn, GFO fiber. This results in chemical attack that shrinks the aramid fiber portion of the packing, causing extra adjustment of the gland. Also, the heat transfer rate through the aramid yarn is not as high as that through the PTFE/graphite composite. Further, the aramid yarn is more abrasive, has greater running friction and wears shafts at a higher rate than the PTFE/graphite composite yarn.
It is therefore a primary object of the invention to reduce extrusion of packing materials while maintaining leakage protection and low friction.
It is a further object of this invention to reduce undesired friction and shrinkage of the packing to a minimum while retaining resistance to extrusion when the packing is installed in rotating equipment.
It is another object of the invention to create a packing that has higher resistance to fluid flow in one direction through the packing-shaft interface than in the opposite direction when installed for this purpose.
It is also an object of the invention to reduce undesired friction between the packing-rod interface during the return stroke when installed in a gravity return hydraulic ram.